RAD 232 IMAGE FORMATION - L2 - 2024 PDF

Image Formation and Radiographic Quality L1 Lecturer : Hanan Alayfei BSc. M.Sc. HI E-mail: [email protected]  To produce a radiographic image, x-ray photons must pass through tissue and interact with an INTRODUCTION image receptor (IR), a device that receives...

Image Formation and Radiographic Quality L1 Lecturer : Hanan Alayfei BSc. M.Sc. HI E-mail: [email protected]  To produce a radiographic image, x-ray photons must pass through tissue and interact with an INTRODUCTION image receptor (IR), a device that receives the radiation leaving the patient. Both the quantity and the quality of the primary x- ray beam affects its interactions within the various tissues that make up anatomic parts. The composition of the anatomic tissues affects the x-ray beam interaction. The absorption characteristics of the anatomic part are determined by its thickness, atomic number of the atoms contained within it, and tissue density or compactness of the cellular structures.  Finally, the radiation that exits the patient is composed of varying energies and interacts with the image receptor to form a latent or invisible image and must be processed. A visible radiographic image is produced following the processing of the latent or invisible image. Depending on the type of imaging system, the acquiring, processing, and displaying of images can vary significantly IMAGE FORMATION: a.Differential Absorption The process of image formation is a result of differential absorption of the x-ray beam as it interacts with anatomic tissue. The term differential is used because varying anatomic parts do not absorb the primary beam to the same degree. Creating a radiographic image by differential absorption requires several processes to occur: Øbeam attenuation, Øabsorption, Øtransmission.  Beam Attenuation As the primary x-ray beam passes through anatomic tissue, it loses some of its energy (intensity). Fewer x-ray photons remain in the beam after it interacts with anatomic tissue. This reduction in the intensity or number of photons in the primary x-ray beam is known as attenuation. Beam attenuation occurs as a result of the photon interactions with the atomic structures that comprise the tissues. Two distinct processes occur during beam attenuation: absorption and scattering  Absorption Complete absorption of the incoming x-ray photon occurs when it has enough energy to remove (eject) an inner-shell electron. The ejected electron is called a photoelectron, and it quickly loses energy by interacting with nearby tissues. The ability to remove (eject) electrons, known as ionization, is a characteristic of x-rays.  Scattering. Some incoming photons are not absorbed but instead lose energy during interactions with the atoms comprising the tissue. This process is called scattering. It results from an interaction between diagnostic x-rays and matter, known as the Compton effect. If a scattered photon strikes the image receptor, it does not contribute any useful information about the anatomic area of interest. If scattered photons are absorbed within the anatomic tissue, they contribute to radiation exposure to the patient. If the scattered photon leaves the patient and does not strike the image receptor, it could contribute to radiation exposure of anyone near the patient.  If a scattered photon strikes the image receptor, it does not contribute any useful information about the anatomic area of interest. If scattered photons are absorbed within the anatomic tissue, they contribute to radiation exposure to the patient. If the scattered photon leaves the patient and does not strike the image receptor, it could contribute to radiation exposure of anyone near the patient. Factors Affecting Beam Attenuation 1. Tissue Thickness: Increasing the thickness of a given anatomic tissue increases beam attenuation by either absorption or scattering. X-rays are exponentially attenuated and are generally reduced by approximately 50% for each 4–5 cm (1.6–2 in) of tissue thickness. More x-rays are needed to produce a radiographic image for a thicker anatomic part Factors Affecting Beam Attenuation 2.Type of Tissue Tissues composed of elements with a higher atomic number, such as bone (which has an effective atomic number of 13.8), attenuates the x-ray beam more than tissue composed of elements with a lower atomic number, such as fat (which has an effective atomic number of 6.3). 3. Tissue Density (matter per unit volume), or the compactness of atomic particles comprising the anatomic part, also affects the amount of beam attenuation. For example, muscle (effective atomic number 7.4) and fat (effective atomic number 6.3) tissue are similar in effective atomic number, but their tissue densities vary. Muscle tissue has atomic particles that are more densely packed or compact and therefore attenuate the x-ray beam more than fat cells. Four substances account for most of the beam attenuation in the human body: bone, muscle, fat, and air. 4. X-ray Beam Quality. The quality of the x-ray beam or its penetrating ability affects its interaction with anatomic tissue. Higher-penetrating x-rays (shorter wavelength with higher frequency) are more likely to be transmitted through anatomic tissue without interacting with the tissues’ atomic structures. Lower-penetrating x-rays (longer wavelength with lower frequency) are more likely to interact with the atomic structures and be absorbed. The kilovoltage selected during x-ray production determines the energy or penetrability of the x-ray photon, and this affects its attenuation in anatomic tissue (Figure 3-7). Beam attenuation decreases with a higher-energy x-ray beam and increases with a lower-energy x-ray beam  Transmission: If the incoming x-ray photon passes through the anatomic part without any interaction with the atomic structures. The combination of absorption and transmission of the x-ray beam provides an image that structurally represents the anatomic part. Because scatter radiation is also a process that occurs during interaction of the x-ray beam and the anatomic part, the quality of the image created is compromised if the scattered photon strikes the image receptor.  Exit Radiation: When the attenuated x-ray beam leaves the patient, the remaining x-ray beam, referred to as exit radiation or remnant radiation, is composed of both transmitted and scattered radiation. The varying amounts of transmitted and absorbed radiation (differential absorption) create an image that structurally represents the anatomic area of interest. Scatter exit radiation (Compton interactions) that reach the image receptor do not provide any diagnostic information about the anatomic area. Scatter radiation creates unwanted exposure on the image called fog.  Anatomic tissues that vary in absorption and transmission create a range of dark and light areas (shades of gray) The various shades of gray recorded in the radiographic image make anatomic tissues visible. Skeletal bones are differentiated from the air-filled lungs because of their differences in absorption and transmission. Less than 5% of the primary x-ray beam interacting with the anatomic part actually reaches the image receptor, and an even lower percentage is used to create the radiographic image The exit radiation interacting with an image receptor creates the latent image, or invisible image. This latent image is not visible until it is processed to produce the manifest image, or visible image.  Less than 5% of the primary x-ray beam interacting with the anatomic part actually reaches the image receptor, and an even lower percentage is used to create the radiographic image. The exit radiation interacting with an image receptor creates the latent image, or invisible image. This latent image is not visible until it is processed to produce the manifest image, or visible image.  * RADIOGRAPHIC QUALITY Radiographic images can be acquired from two different types of image receptors: digital and film-screen. The process of creating a latent image by differential absorption is the same for both digital and film image receptors; however, the acquisition, processing, and display vary greatly.  The visibility of the anatomic structures and the accuracy of their recorded structural lines (sharpness)determine the overall quality of the radiographic image. The visibility of the recorded detail refers to the brightness and contrast of the image, and the accuracy of the structural lines is achieved by maximizing the amount of spatial resolution and minimizing the amount of distortion. Visibility of the recorded detail is achieved by the proper balance of image brightness and contrast.  Image Contrast In addition to sufficient brightness or density, the radiograph must exhibit differences in the brightness levels or densities (image contrast) to differentiate among anatomic tissues. The range of brightness levels is a result of the tissues’ differential absorption of the x-ray photons. An image that has sufficient brightness but no differences appears as a homogeneous object (Figure 3-15). This appearance indicates that the absorption characteristics of the object are equal. When the absorption characteristics of an object differ, the image has varying levels of brightness (Figure 3- 16).  Radiographic contrast is the combined result of multiple factors associated with the: Ø anatomic structure, Øradiation quality, Øimage-receptor capabilities, Øin digital imaging, computer processing and display. Subject contrast refers to the absorption characteristics of the anatomic tissue imaged and the quality of the x-ray beam. Subject contrast is affected by: Differences in tissue thickness Density effective atomic number  Increasing the penetrating power of the x-ray beam: decreases attenuation, reduces absorption, increases x-ray transmission, resulting in fewer differences in the brightness levels recorded in the radiographic image.  Radiographic or image contrast is a term used in both digital and film- screen imaging to describe variations in brightness and density. In digital imaging, the number of different shades of gray that can be stored and displayed by a computer system is termed gray scale. Digital images can be displayed to show a range of gray levels from high to low contrast. High-contrast images display fewer shades of gray but greater differences among them (Figure 3-20). Low-contrast images display a greater number of gray shades but smaller differences among them (Figure 3-21).  The term contrast resolution is used to describe the ability of an imaging receptor to distinguish between objects having similar subject contrast. Digital image receptors have improved contrast resolution compared with film-screen image receptors. A film image with a few visible densities but great differences among them is said to have high contrast; this is also described as short-scale contrast. A radiograph with a large number of densities but few differences among them is said to have low contrast; this is also described as long-scale contrast. Subject contrast: It is a feature of the object (subject) under examination. At a given beam energy, the degree of beam attenuation between anatomical structures is determined by the physical density and atomic number of those structures. Subject contrast will change if the beam energy (kVp) is varied or via the use of a contrast agent, which will change atomic number within an area of the object. X-Radiation passing through the body is attenuated by different amounts by the different thicknesses, densities and atomic numbers of the structures in the body. The beam emerging from the patient varies in intensity: more will emerge if the beam encounters only a small thickness of soft tissue. The difference in intensities in the emergent beam is called subject contrast or radiation contrast. Subject contrast refers to the absorption characteristics of the anatomic tissue imaged and the quality of the x-ray beam. Differences in tissue thickness, density, and effective atomic number contribute to subject contrast  Kilovoltage: at lower kilovoltage, there is a greater difference in attenuation by structures of different density and atomic number than at higher kilovoltage. Therefore, at lower kilovoltage, there is a greater subject contrast. This can be used to advantage when examining areas of low subject contrast, such as the breast. Conversely, there is a high subject contrast within the chest (marked differences in patient density when comparing the lungs and the heart). A higher kilovoltage will therefore reduce this subject contrast and produce a more even image density.